Variable tension devices using torsion members are used in many different circumstances. One such use is as a variable counterbalance device, such as is disclosed in the copending application Ser. No. 477,337 of Mar. 21, 1983 entitled "Counterbalance Device and Torsion Member Usable Therein", with respect to which continuation application Ser. No. 712,757 was filed on Mar. 18, 1985, filed by the inventor herein and assigned to the assignee of this application. The structure of the present invention will be here specifically described in connection with a counterbalance device of that general type, but it will be understood that the torsion device of the present invention is suitable for use in many other environments.
Counterbalance devices of the type used to illustrate the applicability of the device in the present invention are employed where one part is articulately connected to another so as to be movable between first and second positions, the nature of that part being such that its effective weight varies as it thus moves. For example, when a heavy lid is lifted from a horizontal position to a vertical position, it seems heaviest at the beginning of that movement and its effective weight decreases as it moves toward its vertical open position. Hence the force necessary to move it is greatest when the lid is horizontal and least when the lid is vertical, thus creating a tendency to slam the lid into its vertical position and also, because of the effective weight of the lid, tending to cause the lid to slam into its horizontal position. In addition, under normal circumstances, if the lid is released in an intermediate position it will tend to fall back to its horizontal position, usually with considerable force. Consequently, counterbalance devices are employed, which are effective to minimize the differences in apparent weight of the lid as it moves from one operative position to the other, and preferably effective to exert a force on the lid at any given intermediate position which is substantially equal and opposite to the force that the lid would normally exert, so that if the lid is released in an intermediate position it will tend to stay in that position.
Actual physical counterbalancing by means of an added and appropriately located weight involves significant problems of weight, space, cost and safety. Therefore many different artificial counterbalancing devices have been proposed, but few if any can produce perfect or neutral counterbalancing (by neutral counterbalancing is meant counterbalancing such that the lid will remain in whatever position it finds itself, and will exert a smooth, low resistance to movement from that position) without complex and expensive structure, and most take up a significant amount of space and add significant weight to the overall construction. Many such devices utilize hydraulic or pneumatic cylinders, which are subject to significant maintenance problems. Springs, including torsion springs, have been used in the past for this purpose, but they either are extremely bulky, heavy and expensive or they can be used only for relatively light work. Even the bulkiest spring hinge becomes unsuitable when truly heavy lids are involved, and for such heavy lids the art has turned to the use of hydraulic shock absorbers or gas springs, but they are significantly more costly, present very substantial space problems, and obstruct side access when the lid is lifted.
Moreover, torsion spring assemblies as used in the past have involved structures individually designed for a particular application. When a new application calling for even a slightly different torsion characteristic is presented, a new and different structure must be designed for that purpose. This adds greatly to the cost of such devices. Moreover, adjustability of such structures, to accommodate external or internal changes or to make a given structure adaptable for use in a plurality of environments, is difficult and unreliable.
The structure disclosed in the aforementioned application Ser. No. 477,337 was quite effective in achieving its desired objective, but additional experience with that construction has led to an appreciation of certain areas where improvements were appropriate, particularly with regard to cost and reliability of manufacture and adaptability to different specific environments.